Low grade coal (lowly carbonized coal) such as brown coal and subbituminous coal contains much water and the amount of heat generated thereby per unit mass is therefore small and the transportation efficiency thereof is low. Because the reserve of the low grade coal is however abundant, from the viewpoint of the effective use of the resource, the low grade coal is dewatered and is thereafter compression-molded into a specific size to be used as fuel with an increased amount of heat generated thereby per unit mass and an improved handling property.
The low grade coal has a nature that the surface activity thereof is high and the low grade coal tends to react with oxygen, and may therefore spontaneously ignite when the low grade coal is exposed to air during transportation and storage thereof. To prevent the spontaneous ignition, aging is generally conducted therefor according to which a specific amount of air (oxygen) is supplied to the coal after the compression molding and the coal is thereby slowly oxidized to be stabilized. For example, a method has been proposed as a method of the aging, according to which the reactivity of the coal is degraded by applying thereto hydration treatment and oxidization treatment (see Japanese Laid-Open Patent Publication No. 2011-37938).
The coal applied with the aging as above method is crushed to adjust its particle size for its storage. The crushed coal tends to generate dust during the transportation thereof and the like. Adding water to the coal by sprinkling water thereon is effective to prevent the dust generation.
The inventors however found that, when the water was added to the coal after the aging, the spontaneous ignition property was enhanced due to the recovery of the surface activity of the coal. When the water is sprinkled on the coal after the aging, the reaction rate (the oxygen consumption rate) of the coal is increased and the effect of the aging is degraded resulting in enhancement of the spontaneous ignition property. The traditional method has therefore difficulty in simultaneously realizing the dust generation preventive property and the spontaneous ignition preventive property.